This invention relates to electrochromic devices in which light transmittance varies when an electrical field is applied across an electrochromic material contained therein and, more particularly, to an electrochromic assembly adapted to avoid the disadvantages of coloration segregation when such an assembly is operated over prolonged periods of time.
Electrochromic devices, and especially those including a single-compartment, solution-phase, self-erasing electrochromic/electrochemichromic device, typically include an electrochromic or electrochemichromic material confined between a pair of spaced glass plates. The inside surfaces of the glass plates are coated with transparent, electrically conductive material to which electrical energy is applied creating an electrical field across the electrochromic material. Such field causes the material to color, thereby varying light transmittance through the device. Removal of the electric field or reversal of the electric field causes the device to return to its colorless, transparent state.
A known disadvantage of single-compartment, solution-phase, self-erasing electrochromic devices is exhibited whenever such a device is colored for an extended period of time, sometimes for as short as 60 seconds but more usually over several minutes or hours. When first bleached by removal of the electrical energy after prolonged operation or coloration, bleaching in the device is often nonuniform. Bands of color remain adjacent to the electrically conductive bus bars due to voltage gradient induced segregation, a phenomenon related to the depth of coloration associated with maxima in applied electrical potential with resultant potential gradient assisted diffusion of charged molecules. Bands of coloration may also be seen adjacent regions where there exists a significant concentration difference between colored electrochromic molecules and uncolored electrochromic molecules. This effect is termed concentration induced coloration segregation and is particularly noticeable when, in the above type device, one region is colored for a prolonged period while immediately adjacent regions are left uncolored. Differences in solubilities between the colored and uncolored forms of any of the electrochromic species may also contribute to segregation.
The effects of coloration segregation in electrochromic devices have both cosmetic and functional disadvantages. Bands of colors seen after extended coloration can be aesthetically displeasing in devices such as rearview mirrors, windows, office partitions, information displays and the like where users may question whether the device is damaged or working properly. In information display devices where regions of the device are colored while immediately adjacent regions remain uncolored, the functionality of such devices can be impaired because diffusion of colored molecules into adjacent uncolored regions reduces or eliminates lines of demarkation and thus information definition. While this can occur even during the period of prolonged operation, it is particularly evident upon first bleaching after such a period of extended use. To date, these segregation effects have limited the usefulness and commercial success of many electrochromic devices.
Particular applications of electrochromic devices have also been affected by the above segregation disadvantages. For example, in rearview mirrors, where single-compartment, self-erasing, solution-phase electrochromic devices have usually been offered only in an automatic mode operative upon the sensing of bright lights from the rear of a vehicle, remotely operated, manual rearview mirrors using electrochromic principles could provide prolonged coloration periods for 20 minutes to an hour or more such as would occur during normal night driving. Such extended operation would result in coloration segregation which would be displeasing to the driver until the resultant coloration bands diminished due to recombination of the respective electrochromic molecules. Such segregation is also potentially hazardous in that rear vision is diminished in the regions of the mirror affected by the color bands.
In other devices such as building windows, office partitions, security/privacy windows, and the like, and particularly those where prolonged operation to reduce transmitted light is common, any uneven coloration effects would be plainly visible during daytime/high light level usage creating a cosmetically unappealing effect.
For information display devices, individually addressable regions in such a device are often created by defining pixels by means of etching, laser scribing, sandblasting, masking or otherwise separating portions of an electrically conductive coating on at least one of the glass sheets which confine the electrochromic material. During prolonged operation or coloration of any one pixel, adjacent pixels would be affected through coloration segregation effects due to voltage gradients and especially through concentration gradients such that pixel definition would be impaired and the usefulness of the device diminished.
Various attempts have been made to reduce the effects of coloration segregation in electrochromic devices. For example, thickeners have been added to the electrochromic material as a means of reducing segregation. Also, novel solvents have been used which drastically reduce segregation by reducing leakage currents therein. Yet, there continues to be a need for still further improved electrochromic devices in which the effects of segregation are almost totally eliminated to allow electrochromic devices which can be operated substantially indefinitely without adverse effects on performance, functionality and consumer acceptance.